Added tensor_contract function to tensor.py.
Fixes for Python 3 and unit tests.
Added support for colormaps to Hinton diagrams.

This should help provide accessibility support, as the red/blue
colormap can be difficult to impossible to read for colorblind users.
Added superoperator support, colorblind mode to hinton().
Added colorblind_safe setting.
tensor_contract now preserves superrep by default.
Added fallthrough case for to_super to catch bugs elsewhere.
New label support for hinton().
Merge remote-tracking branch 'upstream/master' into feature-tenscontract

Conflicts:
	qutip/operators.py
Added docstrings to _isqubitdims and _cb_labels.
Fixes for qeye/identity w/ dims support.
Fix for color map in _blob.
Merge pull request #268 from cgranade/feature-tenscontract

Arbitrary tensor contraction.